02 XR650R Possible Carb Issue -- Help Please!

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I recently purchased by first BRP. It is a 2002 model "uncorked". I noticed that the bike accelerates beautifully throughout the entire rev range with no misses or hicups. However, if you are on the gas hard and begin to letup, the bike acts like you just completely shut off the throttle and then got right back into it. In effect, the bike dives then comes right back on making it very difficult to ride. Again, this only happens when you let off after hard acceleration.

I took the carb apart today to see what low speed jet was installed. It was the 68S and the main jet is a 175. I was hoping the low speed jet was the stocker, which might have been the culprit, but I believe the bike is jetted correctly. The bowl had quite a bit of varnish so I cleaned up the bowl, blew out the jets, and reinstalled. Unfortunately, the issue is still there and I am unable to find any post with a similar issue.

Any help or suggestions on how to correct this would be greatly appreciated. I can't wait to get out and ride on the trails but want to be sure I have it all sorted first!

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What do you mean by "the bike dives then comes right back on"..Does this mean you're decelerating and suddenly the bikes accelerating again without you touching the throttle,,

Really tight throttle cables,,ie not much slack and a short twist throttle assembly could I suppose make a bike a bit twitchy in throttle control,,either that or the cables or something else carb related is hanging up and the bikes running on after you release the throttle..

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What do you mean by "the bike dives then comes right back on"..Does this mean you're decelerating and suddenly the bikes accelerating again without you touching the throttle,,

Really tight throttle cables,,ie not much slack and a short twist throttle assembly could I suppose make a bike a bit twitchy in throttle control,,either that or the cables or something else carb related is hanging up and the bikes running on after you release the throttle..

Thanks for your response Horri. If I am wide open then let off to about 7/8 throttle, for a split second the bike acts like I just closed the throttle completely then comes back on to the 7/8 throttle position instead of a smooth and gradual decel from full throttle to 7/8 throttle. After the immediate hesitation on decel, the bike acts fine. Sorry, I am doing my best to describe the issue. I have never owned a bike that has ran so well on accel but so poorly on decel. Frustrating!

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I don't know a lot about 650rs,,Has it got the original carb on it??,,the name on it might assist.,I can't say a lot more about it apart from what I have..,short twist throttle assembly (if theres such a beast) ,making the thing twitchy to the touch or hanging cables or slide is about all I can suggest,,

Edited November 20, 2011 by Horri

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I don't know a lot about 650rs,,Has it got the original carb on it??,,the name on it might assist.,I can't say a lot more about it apart from what I have..,short twist throttle assembly (if theres such a beast) ,making the thing twitchy to the touch or hanging cables or slide is about all I can suggest,,

If the slide was hanging it should keep revving not cut out.

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"Varnished up", Unless you have a pilot reamer that pilot jet won't be completely clean.You can spray and blow for days but they still can be slightly resticted with a film, Also was the emultion tube soaked in carb cleaner,and what condition is the decel diaphram?

Re reading again...you say it does it when backed off to 7\8,,this is no where close to the pilot jetting stage. Dissregard what i said before... sounds like a bad decel diaphram of the emultion tube dirty.

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If you mean closed 7/8 throttle, so that only 1/8 is open, look in the venturi on the engine side of the slide. There's a small hole pointed in the direction of the intake manifold. Long time ago I read some trick the said to plug the other side of that hole with RTV silicone and it was supposed to improve the throttle response or something stupid like that. I tried it and all I noticed was a huge amount of engine braking, which is what I think you're talking about. Anyway, I cleaned out all the silicone I put in that hole and everything worked normal again. So in fewer words, check your hole.

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The carb doesn't transition from on/off throttle smoothly but rather abruptly.

It is worse at some speeds than other but it is always there.

It is more noticeable when riding on pavement or hard surfaces where the issue is more pronounced when traction is good.

Richening the low speed jetting, richening the fuel screw helsp a bit but you can never tune it out completely. Have to be smoother with the throttle on initial opening and closing and as you noticed especially when rolling the throttle back on after decelarating.

If you don't mind forking out the cash, the Edelbrock carb solves this problem and is a better performing carb everywhere else too.

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my 2000 has the same problem there is a cover on the shifter side of the carb under it is a diaphram and small spring i heard if you plug the port off it helps the cut out i have not done it yet the job of the diaphram is to help prevent back firing when you let off i googled it a while ago there was a site that had pictures of someone doing this try it and let me know